This invention relates to a printing material for the ink jet printing process and a coating compound for preparing an ink receiving layer for the material.
In ink jet printing systems printing operations are carried out by forming droplets of ink by means of various ink jet methods, such as electrostatic attraction methods, bubble formation processes, etc., and these droplets of ink are applied to a printing material. Such printing processes make it possible to print images with a very high resolution directly from electronic data. The image receiving materials used for this purpose must meet certain high requirements so that the image produced by the ink jet process will:
have a high resolution; PA1 have a high color density; PA1 have sufficient color gradations; PA1 be smudgeproof; and PA1 be waterfast. PA1 the ink must be rapidly absorbed by the printing material; PA1 the droplets of ink sprayed on the printing material must spread out in the most accurate possible manner in the shape of a circle and with precisely defined outlines; PA1 the ink diffusion in the printing material must not be too great so that the diameter of the ink spots does not increase any more than is absolutely necessary; PA1 when one ink spot overlaps with another ink spot which was previously applied, it should not have any negative effect or smear; PA1 the printing material must have a surface that permits a high visual reflection density and a high brilliance of the dyes; and PA1 the printing material should have a high dimensional stability and should not stretch after the printing process.
In order to satisfy these requirements or goals, the following basic conditions must be met:
Some of these requirements are contradictory to each other. For example, if the material becomes smudgeproof too rapidly, there will be little or no spreading of the droplet of ink and, thus, the clarity of the resulting image is impaired. On the basis of the requirements of the printing material, there has been a search for ways to obtain an image with the highest possible ink density, while still being as smudgeproof as possible.
Papers in which the ink fluids can be absorbed in the spaces formed between the pulp fibers in the paper or between the fibers and filler have been used as the printing material for ink jet printing. Another group of printing materials include papers having a special ink receiving layer.
The ink receiving layers consist essentially of a pigment/binder mixture. In addition to increasing the whiteness of the printing material, the pigments serve the function of retention of the dyestuffs from the printing ink on the surface of the sheet. A high pigment concentration leads to a high porosity of the layer (German Patent No. 30 24 205). This makes the paper quite smudgeproof. However, at the same time the dyes are also drawn out of the ink into the interior of the printing material, and this has a negative effect on the color density of the image.
Japanese Patent JP 61-041585 discloses a method of producing printing material with a receiving layer of polyvinyl alcohol and polyvinylpyrrolidone. The mixing ratio of the two components PVA to PVP is 3:1 to 1:5. However, a disadvantage of this material is its inadequate waterfastness and its wet rub off properties.
Japanese Patent JP 61-261089 discloses a transparent material for overhead projectors which contains a cationic conductive resin in addition to a mixture of polyvinyl alcohol and polyvinylpyrrolidone. This makes the paper smudgeproof and waterproof, but the wet rub off properties are not adequate.
Therefore, the object of the present invention is to provide a printing material for the ink jet printing process which will fulfill the requirements mentioned above, but specifically will assure a good waterfastness and wet rub off properties, in addition to a high color density and image definition or clarity.
This object is achieved by means of a printing material comprising a support and an ink receiving layer applied to the support and containing a polyvinyl alcohol, polyvinylpyrrolidone homopolymer and/or vinylpyrrolidone copolymer, and a water soluble substance containing aldehyde groups.
The water soluble substance containing aldehyde groups may be a melamine formaldehyde condensation product containing free CHO groups in the amount of at least 0.05 mol%. However, a polyvalent aldehyde, such as an oxalaldehyde, may also be used.
The quantity ratio of the water soluble substance to the hydrophilic binder present in the ink receiving layer which is a mixture of polyvinyl alcohol, polyvinylpyrrolidone homopolymer and/or polyvinylpyrrolidone copolymer, is 1:1 to 1:40. In a preferred embodiment, the ratio is 1:1.5 to 1:27.
The quantity ratio of polyvinyl alcohol to polyvinylpyrrolidone homopolymer and/or copolymer in the binder mixture is 10:1 to 4:1.
In a special embodiment of the invention, the polyvinylpyrrolidone copolymer is a vinylpyrrolidone vinyl acetate or a vinylpyrrolidone styrene copolymer.
The amount of water soluble substance containing aldehyde groups in the ink receiving layer is 2 to 15 wt %, especially 3 to 10 wt %.
The amount of binder in the layer is 15 to 80 wt %, especially 30 to 60 wt %.
In a special embodiment of the invention, the ink receiving layer contains a quaternary ammonium compound with a certain cationicity which is determined with PCD titration with a 1.times.10.sup.-3 n solution of sodium polyethylenesulfonate (PES solution). The values for the cationicity obtained by this method for the ammonium compounds according to this invention amount to 15 to 30 ml PES solution, especially 20 to 25 ml PES solution. The quaternary ammonium compounds include, for example, polyamine salts and polyamideamine compounds. Polydiallyldimethylammonium chloride has proven to be especially advantageous.
The ink receiving layer according to the invention may also contain other additives, such as white pigments, colored pigments, dyes, dispersants, wetting agents, curing agents and optical brighteners.
Pigments that can be used in the ink receiving layer include silicic acid, clay, zeolites and other inorganic pigments. In a preferred embodiment of this invention, an amorphous silicic acid having a pore volume of 1.0 to 2.5 ml/g with a particle size of .ltoreq.5 .mu.m is contained in the ink receiving layer. The amount of pigment in the ink receiving layer is 10 to 80 wt %, especially 40 to 65 wt %.
The ink receiving layer is applied to the carrier from an aqueous dispersion with the help of any of the conventional methods of application and metered addition. The coating weight of the ink receiving layer is 0.5 to 15 g/m.sup.2, preferably 2 to 8 g/m.sup.2. A plastic film or a coated or uncoated base paper may be used as the support. The base paper may be paper sized with an acidic or neutral size. A base paper sized with diketene and containing 5 to 20 wt % pigment and/or filler, such as TiO.sub.2, CaCO.sub.3 and SIO.sub.2, is preferred.
In another preferred embodiment of the invention, the back side of the base paper has a layer containing a hydrophilic colloidal binder such as starch, modified starch, polyvinyl alcohol or gelatin. An oxidized potato starch is especially preferred.
The layer on the back side may contain up to 60 wt % fillers, such as for example, clay, zeolite, CaCO.sub.3, and pigments and other additives.